Protein kinase A phosphorylation alters Kvβ1.3 subunit-mediated inactivation of the Kv1.5 potassium channel

The human Kv1.5 potassium channel forms the I(Kur) current in atrial myocytes and is functionally altered by coexpression with Kvβ subunits. To explore the role of protein kinase A (PKA) phosphorylation in β-subunit function, we examined the effect of PKA stimulation on Kv1.5 current following coexpression with either Kvβ1.2 or Kvβ1.3, both of which coassemble with Kv1.5 and induce fast inactivation. In Xenopus oocytes expressing Kv1.5 and Kvβ1.3, activation of PKA reduced macroscopic inactivation with an increase in K⁺ current. Similar results were obtained using HEK 293 cells which lack endogenous K⁺ channel subunits. These effects did not occur when Kv1.5 was coexpressed with either Kvβ1.2 or Kvβ1.3 lacking the amino terminus, suggesting involvement of this region of Kvβ1.3. Removal of a consensus PKA phosphorylation site on the Kvβ1.3 NH₂ terminus (serine 24), but not alternative sites in either Kvβ1.3 or Kv1.5, resulted in loss of the functional effects of kinase activation. The effects of phosphorylation appeared to be electrostatic, as replacement of serine 24 with a negatively charged amino acid reduced β-mediated inactivation, while substitution with a positively charged residue enhanced it. These results indicate that Kvβ1.3-induced inactivation is reduced by PKA activation, and that phosphorylation of serine 24 in the subunit NH₂ terminus is responsible.